Printing process using a plurality of drive signal types

ABSTRACT

A maskable drive signal generator selectively generates one of n types of maskable drive signals for each main scan pass, and a drive signal masking section generates a drive signal to be supplied to ink-expulsion drive elements of a print head by masking the maskable drive signal according to a print signal. The printing of ink dots on each raster line is completed in n×m main scan passes while employing each of the n types of maskable drive signals m times on each raster line.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a technique for printing throughemission of ink droplets.

[0003] 2. Description of the Related Art

[0004] In recent years, ink-jet printers, which emit ink onto a printmedium from a print head, have come to enjoy widespread use as outputdevices for computers. While conventional ink-jet printers can onlyreproduce pixels in binary fashion (ON or OFF), multilevel printers thatcan reproduce pixels with three or more levels have been proposed morerecently. Multilevel pixels can be reproduced, for example, bymanipulating the size of the dots formed at pixel locations.

[0005] Producing dots of different sizes requires providing to the driveelements of the print head drive signals of complex waveforms. In actualpractice, it is difficult to generate drive signals having appropriatewaveforms for producing dots of the desired size. Even where a printingdevice uses only one dot size, it is difficult in certain instances togenerate a drive signal having an appropriate waveform for producingdots of the proper size.

[0006] Further, in order to increase the printable area of a printmedium, printing is sometimes performed using a different printingscheme for the top edge and bottom edge portions of the medium than isused in the medial area of the print medium. In such cases, the printingscheme employed for the top edge and bottom edge portion of the mediumwill ideally be conformable to the printing scheme employed for themedial area of the print medium.

SUMMARY OF THE INVENTION

[0007] Accordingly, an object of the present invention is to performprinting with drive signals having appropriate waveforms for producingdots of the desired size.

[0008] Another object of the present invention is to perform printing inthe top edge and bottom edge portions of a print medium by means of aprinting scheme that is conformable to the printing scheme employed forthe medial area of the print medium.

[0009] In a printing apparatus according to the present invention, amaskable drive signal generator selectively generates for each main scanpass any of n types of maskable drive signals where n is an integerequal to 2 or greater. A drive signal masking section generates thedrive signal to be supplied to ink-expulsion drive elements of a printhead, by means of masking the maskable drive signal according to theprint signal on a per-pixel basis. A controller of the printingapparatus executes printing, on at least a part of the print medium,according to a specific printing scheme wherein printing ink dots oneach raster line is completed in n×m main scan passes while employingeach of the n types of maskable drive signals m times on each rasterline where m is an integer equal to 1 or greater.

[0010] In many instances, different maskable drive signals producedifferent print characteristics. Specifically, certain maskable drivesignals can produce dots suitable for higher resolutions, while certainother maskable drive signals can produce dots suitable for higherspeeds. Accordingly, where printing is carried out by means of n typesof maskable drive signals, printing is accomplished with characteristicsrepresenting a combination of the characteristics of each maskable drivesignal.

[0011] In one embodiment, at least one of the n types of maskable drivesignals can effect printing at a print resolution different from thatproduced by other maskable drive signals when used alone for printing;and when a printing operation is performed using the n types of maskabledrive signals, the printing of ink dots and the sub-scan feed arecarried at in units of pixel pitches conforming to a lowest printresolution of the print resolutions achievable by the respective n typesof maskable drive signals.

[0012] In this way, it is possible to effect printing not only with dotsof the lowest print resolution, but with dots of higher resolution(i.e., smaller dots) as well, thereby affording smoother tonereproduction.

[0013] In another aspect of the present invention, when conducting mainscan passes using at least one specific maskable drive signal from amongthe n types of maskable drive signals, a main scan driver conducts themain scan at a speed different from that of main scan conducted usingother maskable drive signals.

[0014] It is permissible for main scan passes employing differentmaskable drive signals to be conducted at different main scan speeds. Byso doing, there is provided a greater degree of freedom during maskabledrive signal generation, so that drive signals having appropriatewaveforms for producing dots of the desired size may be generatedeasily.

[0015] In still another aspect of the present invention, the controllerexecutes printing according to a first printing scheme in a medialsection of a printable area of the print medium, while in at least oneof a leading edge portion and trailing edge portion of the printablearea, executes printing according to a second printing scheme whoseamount of the sub-scan feed is smaller than in the first printingscheme. With respect to raster lines printed according to the firstprinting scheme alone, the controller completes printing in the n×m mainscan passes employing each of the n types of maskable drive signals mtimes. With respect to raster lines printed according to both the firstprinting scheme and the second printing scheme, the controller selectsthe maskable drive signal for each main scan pass according to thesecond printing scheme such that at least n×m main scan passes areperformed employing each of the n types of maskable drive signals atleast m times on each of the raster lines.

[0016] It is frequently the case that different maskable drive signalshave different print characteristics. Specifically, certain maskabledrive signals produce dots at high resolution, while certain othermaskable drive signals produce dots at high speed. Accordingly, whereprinting is carried out by means of n types of maskable drive signals,printing is accomplished with characteristics representing a combinationof the characteristics of each maskable drive signal. According to thisaspect of the present invention, main scanning of raster lines in theedge portions of the printing area is carried out n×m times, employingeach of n types of maskable drive signals at least m times, wherebyprinting in the edge portions of the printing area may be accomplishedwith characteristics representing a combination of the characteristicsof each maskable drive signal, just as in the medial area. That is,according to the present invention, it is possible to perform printingin the edge portions of a print medium by means of a printing schemethat is integrated with the specific printing scheme used in the medialsection of the print medium.

[0017] In one embodiment, the print head is capable of producing aplurality of dot types of different size for at least one ink color on aprint medium using the nozzles, and the print signal contains multiplebits per pixel so as to allow each pixel to be printed in multi tones.Each of the n types of maskable drive signals includes a plurality ofpulses during each pixel interval, and the drive signal masking sectionmasks the maskable drive signals responsive to the multiple-bit printsignal.

[0018] The effect of the invention is particularly great in this casesbecause different maskable drive signals are particularly likely to beused with print heads that can produce dots of different sizes.

[0019] The printing apparatus may perform bidirectional printing wherethe printing of ink dots takes place in both forward and reverse passes.In this case, different maskable drive signals may be selected for theforward pass and the reverse pass of main scan. In addition oralternatively, one of the n types of maskable drive signals may beselected for each main scan pass; and the main scan driver may performeach main scan at a speed appropriate to the selected maskable drivesignal.

[0020] In this way, generation of maskable drive signals and printing bymeans of the same are facilitated.

[0021] Included among the specific aspects of the invention are interalia a printing apparatus and printing method, a computer program forperforming the functions of this apparatus or method, acomputer-readable medium for storing this computer program, and a datasignal containing this computer program and embodied in a carrier wave.

[0022] These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a simplified perspective view of an ink-jet printer 20according to one embodiment of the invention;

[0024]FIG. 2 is a block diagram of the electrical system of printer 20;

[0025]FIG. 3 illustrates the nozzle array provided on the bottom face ofa print head 36;

[0026]FIG. 4 is a block diagram showing the internal structure of thehead driver 63 (FIG. 2);

[0027]FIG. 5 is a block diagram showing the internal structure ofmaskable drive signal generator 304;

[0028]FIG. 6 is a timing chart for generating a maskable drive signalCOMDRV by a maskable drive signal generator 304;

[0029]FIG. 7 illustrates waveform data stored in ROM 310 of maskabledrive signal generation controller 302;

[0030]FIG. 8 is a block diagram of the internal structure of a drivesignal shaping circuit 306;

[0031]FIG. 9 is a timing chart showing a multi-shot dot drive signalwaveform;

[0032]FIG. 10 is a timing chart showing a variable dot drive signalwaveform;

[0033] FIGS. 11(A) and 11(B) are illustrative diagrams comparingmulti-shot dot and variable dot configuration;

[0034]FIG. 12 illustrates printing using both the multi-shot dot seriesand variable dot series;

[0035] FIGS. 13(A) and 13(B) show basic parameters for the ordinaryprinting scheme;

[0036] FIGS. 14(A) and 14(B) show basic parameters for the overlapprinting scheme;

[0037]FIG. 15 illustrates the concept of applying the printing schemesof the embodiment;

[0038] FIGS. 16(A) and 16(B) illustrate the concept of top edge printingof printer paper;

[0039]FIG. 17 shows scanning parameters for medial printing in theembodiment;

[0040]FIG. 18 shows numbers assigned to nozzles used in printing eachraster line during each pass in medial printing;

[0041]FIGS. 19 and 20 show scanning parameters for top edge printing inthe embodiment and the nozzles employed for printing each raster lineduring each pass;

[0042]FIGS. 21 and 22 shows scanning parameters for bottom edge printingin the embodiment and the nozzles employed for printing each raster lineduring each pass; and

[0043]FIGS. 23 and 24 show scanning parameters for a modified version ofbottom edge printing in the present embodiment and the nozzles employedfor printing each raster line during each pass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Preferred embodiments of the invention will be describedaccording to the following order:

[0045] A: Overall Structure of the Apparatus

[0046] B: Internal Strucfure of the Head Driver

[0047] C. Basic Parameters for Normal Print Scheme

[0048] D. Concept of Top Edge printing and Bottom Edge printing

[0049] E. Specific Example of Printing scheme in the Embodiment

[0050] F. Modification Examples

A: Overall Structure of the Apparatus

[0051]FIG. 1 is a simplified perspective view of an ink-jet printer 20according to one embodiment of the invention. The printer 20 comprises apaper stacker 22, a paper feed roller 24 driven by a step motor (notshown), a platen 26, a carriage 28, a step motor 30, a loop belt 32driven by step motor 30, and a guide rail 34 for carriage 28. Thecarriage 28 carries a print head 36 having a plurality of nozzles.

[0052] Printer paper P is taken up from the paper stacker 22 by thepaper feed roller 24 and advanced over the surface of the platen 26 inthe sub-scanning direction. The carriage 28 is towed by the loop belt 32driven by the step motor 30, and moves in the main scanning directionalong the guide rail 34. The main scanning direction is perpendicular tothe sub-scanning direction.

[0053]FIG. 2 is a block diagram of the electrical system of the printer20. The printer 20 comprises a reception buffer memory 50 for receivingsignals transmitted from a host computer 100, an image buffer 52 forstoring print data, and a system controller 54 for controlling alloperations of the printer 20. The system controller 54 is connected to amain scan driver 61 for driving the carriage motor 30, an sub-scandriver 61 for driving the paper feed motor 31, and a head driver 63 fordriving the print head 36.

[0054] A printer driver (not shown) provided in the host computer 100sets various parameters defining a print operation on the basis of auser-specified printing scheme (described later). The printer driveralso generates, on the basis of these parameters, print data forprinting in the specified printing scheme, and sends the print data tothe printer 20. The print data is temporarily held in the receptionbuffer memory 50. In the printer 20, the system controller 54 reads therequired information from the print data stored in the buffer memory 50,and on the basis thereof sends control signals to the drivers 61, 62,and 63.

[0055] The image buffer 52 stores image data for a plurality of inkcolors, produced by separating for each color the print data received inthe reception buffer memory 50. The head driver 63, responsive to acontrol signal from the system controller 54, reads the image data foreach color from the image buffer 52 and in response thereto drives anozzle array for each color provided to the print head 36. The headdriver 63 can generate drive signals having a plurality of differentwaveforms. The internal structure and operation of the head driver 63will be described in greater detail later.

[0056]FIG. 3 shows the nozzle array provided on the bottom face of theprint head 36. The bottom face of the print head 36 has formed therein ablack ink nozzle group K_(D) for emitting black ink, a dark cyan inknozzle group C_(D) for emitting dark cyan ink, a light cyan ink nozzlegroup C_(L) for emitting light cyan ink, a dark magenta ink nozzle groupM_(D) for emitting dark magenta ink, a light magenta ink nozzle groupM_(L) for emitting light magenta ink, and a yellow ink nozzle groupY_(D) for emitting yellow ink.

[0057] The initial capital letter in the symbols for the nozzle groupsindicates the ink color. A subscripted “_(D)” indicates ink ofrelatively high color density and a subscripted “_(L)” indicates ink ofrelatively low color density.

[0058] The plurality of nozzles of each nozzle group are arranged at agiven nozzle pitch k in the sub-scanning direction SS. The nozzle pitchk is an integral multiple of a print resolution (also called a dotpitch) in the sub-scanning direction. Each nozzle is provided with apiezo-electric element (not shown), a drive element that drives thenozzle so as to emit droplets of ink. During printing, ink droplets areemitted from the nozzles as the print head 36 moves in the main scanningdirection MS together with the carriage 28 (FIG. 1).

B: Internal Structure of the Head Driver

[0059]FIG. 4 is a block diagram showing the internal structure of thehead driver 63 (FIG. 2). The head driver 63 comprises a maskable drivesignal generation controller 302, a maskable drive signal generator 304,and a drive signal shaping circuit 306.

[0060] The maskable drive signal generator 304 comprises RAM 320 forstoring a slope value Δj that indicates the slope of the waveform of themaskable drive signal COMDRV. Slope value Δj is used to generate amaskable drive signal COMDRV of arbitrary slope. The maskable drivesignal generation controller 302 comprises ROM 310 (or PROM) in which isstored a plurality of slope values Δj for the forward and reverse passesof main scanning. Drive signal shaping circuit 306 generates a drivesignal DRV by partially or completely masking the maskable drive signalCOMDRV in response to the value of a serial print signal PRT suppliedfrom the image buffer 52 (FIG. 2). This drive signal is presented to thepiezo-electric elements 308, which are the drive elements of thenozzles.

[0061]FIG. 5 is a block diagram showing the internal structure of themaskable drive signal generator 304. In addition to RAM 320, themaskable drive signal generator 304 comprises a first latch circuit 321,an adder 322, a second latch circuit 323, a D/A converter 324, a voltageamplifier 325, and a current amplifier 326. These circuit elements areconnected in series in the order recited here.

[0062] RAM 320 can store up to 32 slope values Δ0˜Δ31. To write a slopevalue Δj to RAM 320, data indicating slope value Δj and an address aresent to RAM 320 by the maskable drive signal generation controller 302.To read a slope value Δj from RAM 320, the address increment terminal ofRAM 320 is presented with an address increment signal ADDINC by themaskable drive signal generation controller 302. A slope value Δj outputby RAM 320 is held in the first latching circuit 321 in response to apulse of a clock signal CLK1. A pulse of the clock signal CLK1 is issuedat given delay interval after the address increment signal ADDINC. Thus,each time the slope value Δj output by RAM 320 is updated, the new slopevalue j is held by the first latching circuit 321.

[0063] The second latching circuit 323 holds the output of the adder 322for the duration of a given cycle in response to the second clock signalCLK2 pulses issued at given cycles. The adder 322 adds the slope valueΔj held by the first latching circuit 321 with the result of theprevious adding operation currently being held by the second latchingcircuit 323. This new add operation result is then held by the secondlatching circuit 323 in response to the subsequent pulse of the secondclock signal CLK2. In other words, the adder 322 and the second latchingcircuit 323 function as an accumulator for sequentially accumulatingslope values Δj at given intervals. The output of the second latchingcircuit 323 shall hereinafter be referred to as “drive signal level dataLD” or simply as “level data LD.” Analog signals output from the D/Aconverter 324 are amplified by the voltage amplifier 325 and the currentamplifier 326 to generate a maskable drive signal COMDRV.

[0064]FIG. 6 is a timing chart for generating maskable drive signalCOMDRV by the maskable drive signal generator 304. When RAM 320 ispresented with an initial pulse of the address increment signal ADDINC(FIG. 6(d)), a first slope value Δ0 is read from RAM 320, held by thefirst latching circuit 321, and input to the adder 322. In FIG. 6, theaddress increment signal ADDINC and the first clock signal CLK1 areshown as being the same, but in actual practice a pulse of the firstclock signal CLK1 will occur with a given delay after a pulse of theaddress increment signal ADDINC.

[0065] Until presented with the next pulse of the address incrementsignal ADDINC, the first slope value Δ0 is repeatedly incremented eachtime that the second clock signal CLK2 rises, thereby producing thelevel data LD. When RAM 320 is presented with the next pulse of theaddress increment signal ADDINC, a second slope value Δ1 is read fromRAM 320, held by the first latching circuit 321, and input to the adder322. In other words, the address increment signal ADDINC (and the firstclock signal CLK1) are signals that generate a single pulse each timewhen a number of pulses of the second clock signal CLK2 becomes equal toa predetermined adding operation count nj (j=1˜30) for the slope valueΔj.

[0066] By using a slope value Δj of zero, the level of the maskabledrive signal COMDRV can be held to horizontal whereas, by using negativeslope value Δj, the level of maskable drive signal COMDRV can bedropped. Accordingly, by setting the values of slope values Δj and theadding operation count nj, it is possible to produce various maskabledrive signals COMDRV of desired waveforms.

[0067]FIG. 7 shows waveform data stored in ROM 310 of the maskable drivesignal generation controller 302. ROM 310 stores a plurality of slopevalues Δj and their adding operation counts nj for each of a pluralityof drive signal waveforms. During the intervals between forward andreverse passes of the main scanning (i.e., the periods for which thecarriage 28 is positioned at the edge section of printer 20, outside theprintable area), the maskable drive signal generation controller 302performs an operation wherein a plurality of slope values Δj to be usedduring the subsequent forward or reverse pass are written to RAM 320 ofthe maskable drive signal generator 304. The adding operation counts njare used in generating the address increment signal ADDINC and the firstclock signal CLK1 in the maskable drive signal generation controller302. The use of the maskable drive signal generator 304 depicted inFIGS. 4 to 7 allows a plurality of maskable drive signals COMDRV havingarbitrary waveforms to be generated selectively for each main scan.

[0068]FIG. 8 is a block diagram of the internal structure of the drivesignal shaping circuit 306. The drive signal shaping circuit 306comprises a shift register 330, a data latch 332, a mask signalgenerator 334, a mask pattern register 336, and a masking circuit 338.The shift register 330 converts the serial print signals PRT suppliedfrom the image buffer 52 into 2-bit 48-channel parallel data. Here, eachchannel represents a signal for one nozzle. A print signal PRT for onepixel for one nozzle is composed of two bits, a most significant bit DHand a least significant bit DL. Mask signal generator 334 generates1-bit mask signals MSK(i) (i=1˜48) for each channel in response to maskpattern data V0˜V3 provided by the mask pattern register 336 and 2-bitprint signals PRT(DH, DL) for each channel,. The masking circuit 338 isan analog switching circuit that, in response to the mask signalsMSK(i), partially or completely masks the waveform of the maskable drivesignal COMDRV for a single pixel interval. As used herein, “masking themaskable drive signal” refers to turning on or off the connection of thesignal line of the maskable drive signal COMDRV to the piezo-electricelement.

[0069]FIG. 9 is a timing chart showing a first drive signal waveformused in the present embodiment. As shown in FIG. 9(A), a first maskabledrive signal COMDRV1 is a signal that produces three identical pulses W1during a single pixel interval. As shown in FIG. 9(B), (C), and (D), allpulses except the first pulse are masked to print a small dot, the thirdpulse is masked to print a medium dot, leaving the first and secondpulses, and the entirety of the maskable drive signal COMDRV1 is usedwithout any masking to print a large dot. By performing masking in thisway for each pixel in response to serial print signals PRT, selectiveprinting of a dot of any of three possible sizes at each pixel locationis possible. The three types of dots produced by this first drive signalwaveform shall hereinafter be referred to as “multi-shot dots.”

[0070]FIG. 10 is a timing chart showing a second drive signal waveformused in the present embodiment. As shown in FIG. 10(A), in the secondmaskable drive signal COMDRV2, each single pixel interval is dividedinto three sub-intervals, with pulses W11, W12, and W13 having threedifferent waveforms being generated in these sub-intervals,respectively. As shown in FIG. 10(B), (C), and (D), all pulses exceptthe second pulse W12 are masked to print a small dot, all pulses exceptthe first pulse W11 are masked to print a medium dot, and all pulsesexcept the third pulse W13 are masked to print a large dot. Byperforming masking in this way for each pixel in response to the serialprint signals PRT, selective printing of a dot of any of three possiblesizes at each pixel location is possible. The three types of dotsproduced by this second drive signal waveform shall hereinafter bereferred to as “variable dots.”

[0071] FIGS. 11(A) and 11(B) are illustrative diagram comparingmulti-shot dot and variable dot configuration. As shown in FIG. 11(A),the smallest multi-shot dot MS is produced with an ink droplet of 13 ng(nano grams), the medium dot MM with 26 ng, and the large dot ML with 40ng. Where only the three types of multi-shot dots MS, MM, and ML areused, print resolution in both the main scanning direction and thesub-scanning direction will be relatively low (360 dpi), but print speedwill be relatively fast. The print resolution achievable using a singledrive signal waveform in this way shall hereinafter be termed “printresolution in simplex mode.”

[0072] As shown in FIG. 11(B), the smallest variable dot VS is producedwith an ink droplet of 4 ng, the medium dot VM with 7 ng, and the largedot VL with 11 ng. Simplex mode print resolution using the variable dotsis 1440 dpi in the main scanning direction and 720 dpi in thesub-scanning direction. The variable dots have the advantage of beingable to print high quality images at higher resolution than withmulti-shot dots. Even where printing is performed with the variable dotsexclusively, it is difficult to print dots at 1440 dpi resolution in themain scanning direction during a single main scan pass. Accordingly, inactual practice, printing of all the dots on a single raster line isaccomplished in four main scan passes. In other words, during eachindividual main scan, dots are printed on each raster line in a ratio ofone of every four pixels, the dots printed during four main scan passessupplementing each other to effect complete printing on each rasterline. The variable dots perform printing at lower speeds than themulti-shot dots, but afford higher resolution.

[0073] The term “multi-shot dot series” shall be used to refercollectively to the three kinds of multi-shot dots MS, MM, and ML, andthe term “variable dot series” shall be used to refer collectively tothe three kinds of variable dots VS, VM, and VL.

[0074]FIG. 12 is an illustrative diagram of printing conducted usingboth the multi-shot dot series and variable dot series. Where both dotseries are used in printing, print resolution in the sub-scanningdirection will be the lower of the two simplex mode print resolutions(namely, the multi-shot dot series print resolution).

[0075] Where the two dot series are used concomitantly, the multi-shotdot series and variable dot series may be overlaid on a same givenraster line. The multi-shot dot series, when used for a given rasterline, may target for printing all pixel locations on the raster line,and the variable dot series, used for the same raster line, may likewisetarget for printing all pixel locations on the raster line. However, inactual practice, superposition of two or more dots on a given pixellocation results in unstable reproduction of image density. Accordingly,image processing by the printer driver in the computer 100 is preferablycarried out in such a way that only one dot is printed at each singlepixel location. As will be apparent from the preceding description, theterm “overlay” is not limited to the narrow sense of actually printingtwo or more dots at the same exact pixel location, but includes thebroader meaning of targeting the same pixel location for printing. Theterm “target a pixel location for printing” is used in the sense of“producing a state wherein a dot may be printed at a pixel location bymeans of driving the drive element.”

[0076] By overlaying the multi-shot dot series and the variable dotseries on each raster line, it becomes possible to print using dots ofsix different sizes. The multi-shot dot series is predominantly used inareas of high image density, while the variable dot series tends to beused in areas of low image density. Thus, in areas of low image density,it is possible to reduce dot granularity in substantially the samemanner as when using the variable dot series exclusively. Where the twodot series are used concomitantly, an image can be reproduced using dotsof six different sizes, affording better image quality than is the casewhere the multi-shot dot series is used alone.

[0077] The smallest dot MS of the multi-shot dot series is of 13 ng,while the largest dot VL of the variable dot series is of 11 ng, andthus the two dots are formed using about equal amounts of ink. Thus,when using two different dot series, by setting the size of the largestdot of the smaller dot series to about the same size as the smallest dotof larger dot series in this way, it is possible to achieve smootherhalftone reproduction during printing using both dot series.

[0078] When the variable dot series is employed in printing, mainscanning speed (carriage speed) is lower than main scanning speed whenprinting multi-shot dots alone. The reason is that the waveform of thevariable dot maskable drive signal COMDRV2 (FIG. 10(A)) is more complexthan the waveform of the multi-shot dot maskable drive signal COMDRV1(FIG. 9(A)), and thus a single pixel interval of the drive waveformrequires more time. By way of an example, in variable dot seriesprinting, main scanning speed is about 200 cps (characters per second),whereas in multi-shot dot series printing, main scanning speed is about250 cps. Where both dot series are used concomitantly, average mainscanning speed is about 225 cps, which is lower than the speed for themulti-shot dot series simplex mode. Thus, printing speed is somewhatslower as well.

[0079] As noted, where the variable dot series is used alone, sub-scanresolution is 720 dpi, with printing of all the dots on each raster linebeing completed in four main scan passes. Thus, printing speed is fairlylow. When both dots series are used concomitantly, on the other hand,sub-scan resolution is 360 dpi, with printing of all the dots on eachraster line being completed in two main scan passes. Thus, printingspeed is higher, close to that obtained using the multi-shot dot seriesalone. In low-resolution image areas, image quality is close to thatachieved using the variable dot series alone. Thus, the concomitant useof both dot series affords both high printing speed close to thatobtained using the multi-shot dot series alone, and high image qualityclose to that achieved using the variable dot series alone.

C. Basic Parameters for Ordinary Printing Scheme

[0080] Before proceeding to a detailed description of the printingscheme used in the embodiments of the invention, the basic parametersfor the ordinary printing scheme will be described.

[0081] FIGS. 13(A) and 13(B) show basic parameters for the ordinaryprinting scheme. FIG. 13(A) depicts an exemplary sub-scan using fournozzles, and FIG. 13(B) gives parameters for this printing scheme. InFIG. 13(A), the numbered solid circles indicate the positions of thefour nozzles during each pass in the sub-scanning direction. Here,“pass” refers to one main scan. The numbers in the circles are thenumbers assigned to the nozzles. The positions of the four nozzles movein the sub-scanning direction at the end of each main scan pass. Inactual practice, feed in the sub-scanning direction is accomplished bymoving the paper through the action of the paper feed motor 31 (FIG. 2).

[0082] As indicated at the left part of FIG. 13(A), sub-scan feed L inthis example is a constant value equal to four dot pitches. Thus, duringeach subscan feed, the positions of the four nozzles move fourdot-pitches in the sub-scanning direction. During each main scan thenozzles target for printing all of the dot locations (pixel locations)on each raster line. The number of main scan passes performed on eachraster line (main scan line) is termed the “scan iteration count s.”

[0083] The numbers assigned to the nozzles printing the dots on eachraster line are indicated at the right part of FIG. 13(A). The rasterlines indicated by broken lines extending rightward (the main scanningdirection) from the circles that indicate nozzle positions in thesub-scanning direction are those for which the raster line located aboveand/or below is not printable, so in actual practice printing isprohibited there. On the other hand, the raster lines indicated by solidlines extending in the main scanning direction lie within a range suchthat both the preceding and following raster lines can be printed withdots. This actual printable range is termed the “effective printablerange” or “printable area”.

[0084] Various parameters relating to this printing scheme are given inFIG. 13(B). Printing scheme parameters include nozzle pitch k (in unitof: dots), number of working nozzles N, scan iteration count s,effective number of nozzles Neff, and sub-scan feed L (in unit of dots).

[0085] In the example depicted in FIGS. 13(A) and 13(B), nozzle pitch kis 3 dots. The number of working nozzles N is 4. The number of workingnozzles N refers to the number of nozzles that are actually used, out ofthe plurality of nozzles provided in the head. The scan iteration counts indicates the number of main scan passes performed on each raster lineto complete dot printing. For example, where the scan iteration count sis 2, each raster line would be scanned twice in the main scan directionto complete dot printing. Typically, in such a case, dots are producedat one-dot intervals during each of the two single main scan passes. Inthe example shown in FIGS. 13(A) and 13(B), the scan iteration count sis 1. The effective number of nozzles Neff is equal to the number ofworking nozzles N divided by the scan iteration count s. The effectivenumber of nozzles Neff may be thought of as the net number of rasterlines for which printing is completed in a single main scan.

[0086] The table given in FIG. 13(B) gives the sub-scan feed L duringeach pass, the cumulative value thereof ΣL, and the nozzle offset F.Nozzle offset F is a value indicating the distance (expressed in numberof dots) of nozzle position from a reference position in thesub-scanning direction during each subsequent pass, this referenceposition being defined as the cyclically recurring position (in FIG.13(A), positions four dots away) of a nozzle at which the offset duringthe initial pass is deemed to be zero. For example, as shown in FIG.13(A), after pass 1, nozzle position moves in the sub-scanning directionby a distance equal to the sub-scan feed L (4 dots). Nozzle pitch k is 3dots. Accordingly, during pass 2, the nozzle offset F is 1 (see FIG.13(A)). Similarly, during pass 3, the nozzle position moves ΣL=8 dotsfrom the initial position, so the offset F is 2. Nozzle position duringpass 4 moves ΣL=12 dots from the initial position, so the offset F is 0.Since after three sub-scan feeds the nozzle offset F returns to 0 inpass 4, three sub-scans are designated as one cycle. By repeating thiscycle, dots can be printed on all of the raster lines lying within theeffective printable area.

[0087] As will be apparent from the example of FIGS. 13(A) and 13(B),when nozzle position is located away from the initial position by adistance equal to an integral multiple of the nozzle pitch k, the offsetF is considered to be 0. The offset F is given by the remainder (ΣL)% kobtained by dividing the cumulative value ΣL for sub-scan feed L by thenozzle pitch k. Here, % is an operator indicating that the divisionremainder is taken. If the initial position of the nozzle is thought ofas a cyclically recurring position, the offset F can be viewed as the“phase shift” relative to initial nozzle position.

[0088] Where the scan iteration count s is 1, it is necessary for thefollowing conditions to be met in order to avoid dropout or overlay ofthe raster lines in the effective printed area targeted for printing.

[0089] Condition c1: The number of sub-scan feeds in one cycle equalsthe nozzle pitch k.

[0090] Condition c2: During a single cycle, nozzle offset F after eachsub-scan feed assumes a different value within the range 0˜(k−1).

[0091] Condition c3: The average sub-scan feed (ΣL/k) equals the numberof working nozzles N. In other words, the cumulative value ΣL forsub-scan feed L per cycle is equal to the product of the number ofworking nozzles N and nozzle pitch k, (N×k).

[0092] The above conditions may be understood by considering thefollowing. Since (k−1) raster lines are present between adjacentnozzles, the number of sub-scan feeds per cycle must be equal to k inorder to effect printing of these (k−1) raster lines during one cyclebefore the nozzles return to reference position (position at whichoffset is zero). If the number of sub-scan feeds per cycle is less thank, dropout of raster lines will occur, whereas if the number of sub-scanfeeds per cycle exceeds k, some of raster lines will be printed morethan once. Thus, the aforementioned first condition c1 holds.

[0093] Where the number of sub-scan feeds per cycle is equal to k,printed raster lines will be free from dropout and overlay only wherenozzle offset F after each sub-scan feed assumes a different valuewithin the range 0˜(k−1). Thus, the aforementioned first condition c2holds.

[0094] Where the aforementioned first and second conditions are met,during one cycle, each of the N nozzles prints k raster lines. Thus, N×kraster lines are printed during one cycle. If the third condition c3 ismet, nozzle position after one cycle (i.e., after k sub-scan feeds) willbe away from initial nozzle position by a distance equal to N×k rasterlines, as shown in FIG. 13(A). Thus, where the first through thirdconditions c1˜c3 are met, the printed raster lines will be free fromdropout and overlay over the range of these N×k raster lines.

[0095] FIGS. 14(A) and 14(B) show basic parameters for a printing schemewherein the scan iteration count s is 2 or greater. Where the scaniteration count s is 2 or greater, the same given raster line undergoess main scan passes. The printing scheme wherein the scan iteration counts is 2 or greater shall hereinafter be referred to as “overlap scheme.”

[0096] The printing scheme depicted in FIGS. 14(A) and 14(B) isdifferent in the scan iteration count s and sub-scan feed L from theparameters for printing scheme depicted in FIG. 13(B). As will beapparent from FIG. 14(A), sub-scan feed L in the printing schemedepicted in FIGS. 14(A) and 14(B) is a constant value equal to two dots.In FIG. 14(A), nozzle positions during even-numbered passes areindicated by diamond shapes. Typically, the positions of dots printedduring even-numbered passes are shifted by one dot-pitch in the mainscanning direction relative to the positions of dots printed duringodd-numbered passes, as shown at the right part of FIG. 14(A). Thus, theplurality of dots on a given raster line are printed in intermittentfashion by two different nozzles. For example, the uppermost raster lineof the effective printable area is printed intermittently at one-dotintervals by nozzle 2 during pass 2, and then is printed intermittentlyat one-dot intervals by nozzle 0 during pass 5. In the overlap scheme,each nozzle is driven at intermittent timing so as to prohibit printingof (s−1) dots every time after printing one dot during one main scan.

[0097] The overlap scheme wherein intermittent pixel locations on araster line are targeted for printing during each main scan is termed“intermittent overlap scheme.” Alternatively, all pixel locations on araster line may be targeted for printing during each main scan, ratherthan targeting intermittent pixel locations for printing. That is,overlaid printing of dots at a same given pixel location in the courseof s main scan passes of a single raster line is permitted. This overlapscheme is termed “overlaid overlap scheme” or “complete overlap scheme.”

[0098] In the intermittent overlap scheme, the positions of theplurality of nozzles printing a given raster line are shifted in themain scanning direction, so there are various possibilities as regardsthe actual amount of shift in the main scanning direction during mainscan passes, apart from the configuration depicted in FIG. 14(A). Forexample, it would be possible during the pass 2 to not perform shiftingin the main scanning direction so as to print dots at locationsindicated by the circles, and during pass 5 to perform shifting in themain scanning direction so as to print dots at locations indicated bythe diamonds.

[0099] The bottom row of the table in FIG. 14(B) gives the values of theoffset F for each pass during one cycle. Each cycle is composed of sixpasses, with the offset F for each pass from pass 2 to pass 7 cyclingtwice through the range 0˜2. The change in offset F during the threepasses from pass 2 to pass 4 is the same as the change in offset Fduring the three passes from pass 5 to pass 7. As shown at the left partin FIG. 14(A), the six passes of one cycle can be divided into twosub-cycles each composed of three passes. Each cycle is complete whensub-cycles have been repeated s times.

[0100] Where the scan iteration count s is an integer equal to 2 orgreater, the first to third conditions c1˜c3 described earlier may berewritten as conditions c1′˜c3′.

[0101] Condition c1′: The number of sub-scan feeds in one cycle equalsthe product of nozzle pitch k and scan iteration count s, i.e., (k×s).

[0102] Condition c2′: During a single cycle, nozzle offset F after eachsub-scan feed assumes a different value within the range 0˜(k−1), witheach value being repeated s times.

[0103] Condition c3′: The average sub-scan feed (ΣL/(k×s)) equals theeffective number of nozzles Neff (=N/s). In other words, the cumulativevalue ΣL for sub-scan feed L per cycle equals the product of theeffective number of nozzles Neff and the sub-scan feed count (k×s),i.e., Neff×(k×s).

[0104] The above conditions c1′˜c3′ also hold where the scan iterationcount s is 1. Thus, conditions c1′˜c3′ are generally true for theprinting scheme regardless of the value of the scan iteration count s.That is, where conditions c1′˜c3′ are met, printed dots will be freefrom dropout and unwanted overlay within the effective printing area.Where the intermittent overlap scheme is employed, an additionalrequirement is that the positions of the nozzles printing a given rasterline be mutually shifted in the main scanning direction. Where theoverlaid overlap scheme is employed, it is sufficient simply to fulfillconditions c1′˜c3′, targeting all pixel locations for printing duringeach pass.

D. Concept of Top Edge Printing and Bottom Edge Printing

[0105]FIG. 15 is an illustrative diagram illustrating the concept ofimplementing the printing schemes of the embodiment. A printable area PAin which printing is actually performed is established on printer paperP. For the middle area of printable area PA, a printing scheme formedial printing is employed. This medial printing scheme meetsconditions c1′˜c3 described earlier, and the printing scheme is designedto avoid dropout and unwanted superposition of the printed dots. At thetop edge and bottom edge of printable area PA are respectively employedprinting schemes for top edge printing and for bottom edge printing. Thespecial printing process employed for the top edge of printer paper istermed “top edge printing” and the special printing process employed forthe bottom edge of printer paper is termed “bottom edge printing.”

[0106] FIGS. 16(A) and 16(B) illustrate the concept of the printingprocess employed for the top edge of printer paper. For convenience, theexamples mostly assume that the scan iteration count s is 1.

[0107] As shown in FIGS. 13(A) and 13(B) described earlier, an area inwhich effective printing is not possible (non-printable area) is presentat the top edge of the paper. For top edge printing, the sub-scan feedis set to a value smaller than the feed used during medial printing, inorder to reduce the non-printable area and expand the effective printingarea. Specifically, in the top edge printing depicted in FIGS. 16(A) and16(B), the sub-scan feed L is set to two dot-pitches, a value smallerthan the sub-scan feed L of four dot-pitches used in the ordinaryprinting scheme depicted in FIGS. 13(A) and 13(B). It will be apparentthat, as a result, the effective printing area is expanded by fourraster lines relative to that in FIG. 13(A).

[0108] In pass 4 in FIG. 16(A), nozzle 0 and nozzle 1 do not performprinting. The reason is that the raster lines targeted for printing bynozzle 0 and nozzle 1 in pass 4 have already been targeted for printingby nozzle 2 and nozzle 3 in pass 1.

[0109]FIG. 16(B) gives scanning parameters for top edge printing. Thesescanning parameters do not meet conditions c1′˜c3 in the ordinaryprinting scheme described earlier. The reason is that in top edgeprinting, overlaying of raster lines targeted for printing by workingnozzles is permissible, as shown in FIG. 16(A).

[0110] Typically, in the printing scheme employed for top edge printing,the sub-scan feed value is smaller than in the printing scheme employedfor the middle area of the paper (the area of the paper excluding thetop edge and the bottom edge), thereby expanding the effective printablearea. Bottom edge printing similarly employs a smaller sub-scan feedvalue than in the printing scheme employed for the middle area of thepaper, thereby expanding the effective printable area. Since the conceptof bottom edge printing is substantially identical to that of top edgeprinting and will be well understood by one of ordinary skill in theart, further discussion is not made here.

[0111] In some instances, variable feed (i.e., sub-scan feed with anumber of different feed amounts) is employed in the middle area.Variable feed may be employed in top edge printing and bottom edgeprinting as well. In such cases, the average sub-scan feed for top edgeprinting will be smaller than the average sub-scan feed for medialprinting. The same is true of bottom edge printing. The expression“smaller sub-scan feed” in used in a broad sense to include cases suchas these.

E. Specific Example of Printing Scheme in the Embodiment

[0112]FIG. 17 shows scanning parameters for medial printing in theembodiment. This printing scheme is a overlaid overlap scheme whereinnozzle pitch k is 3, scan iteration count s is 2, and the number ofworking nozzles N is 46.

[0113] The table at the bottom of FIG. 17 gives parameters relating topass 1 through pass 7. The drive signal waveforms for multi-shot dotsand variable dots are used alternately at each pass; multi-shot dotwaveform is used in the forward pass and variable dot waveform is usedin the reverse pass. The sub-scan feed L is a constant value of 23dot-pitches. This medial printing scheme is an overlaid overlap schemewherein the scan iteration count s is 2. Variable feed may used for thesub-scan feed in place of the constant feed.

[0114]FIG. 18 indicates the ordinal numbers of the nozzles responsiblefor printing on each raster line during each pass in medial printing.The “raster numbers” in FIG. 18 are numbered beginning at the top edgeof accessible range for all nozzles of the print head 36, which includesa non-printable range (FIGS. 13(A), 14(A)). The raster number assignmentassumes that top edge printing is not used. For convenience, the top 60raster lines have been omitted from the drawing. An “X” drawn through anumbered cell indicates that the corresponding nozzle is not used. [Inthe area] extending from the top edge through raster line 69, [eachraster] can be main scanned only once, so this represents a nonprintablearea. Each raster line in the effective printing area is main scannedonce with the multi-shot dot series and once with the variable dotseries. Pass 6 and subsequent passes are not depicted.

[0115]FIGS. 19 and 20 show scanning parameters for top edge printing inthe embodiment and the nozzles employed for printing each raster lineduring each pass. As indicated in the table in FIG. 19, pass 1 throughpass 6 represent top edge printing, and pass 7 and subsequent passesrepresent medial printing. A constant value of five dot-pitches isemployed as the sub-scan feed L for top edge printing.

[0116]FIG. 19 shows raster lines 1 to 49 and FIG. 20 shows raster lines49 to 95. In pass 1 through pass 6 (the main scan passes for top edgeprinting) only some of the 46 nozzles employed in medial printing areused.

[0117] As shown in FIG. 19, through the use of top edge printing, rasterline 24 and subsequent lines now lie within the effective printablearea. From comparison with FIG. 18 it will be apparent that theeffective printable area has been expanded by 46 raster lines throughtop edge printing.

[0118] As shown in FIG. 20, each raster line is serviced once with themulti-shot dot series and once with the variable dot series, regardlessof whether printing thereof has taken place during top edge printing orduring medial printing. In other words, the drive signal waveform usedfor printing of a given raster line during top edge printing is selectedsuch that this drive signal waveform differs from the drive signalwaveform used for printing of this same raster line during medialprinting. That is, raster lines that are printed using the multi-shotdot drive signal waveform during medial printing are printed using thevariable dot drive signal waveform during top edge printing. Conversely,raster lines that are printed using the variable dot drive signalwaveform during medial printing are printed using the multi-shot dotdrive signal waveform during top edge printing.

[0119] Certain raster lines may in some instances undergo two passeswith the same drive signal waveform. For example, the multi-shot dotdrive signal waveform is used for raster lines 49, 52, 55 in FIG. 20during pass 1 and pass 7. In pass 1, however, the nozzles that scanthese raster lines are not actuated. In other words, according to theembodiment, the drive signal waveform for each pass is selected in sucha way that each raster line is serviced at least once with each of thetwo drive signal waveforms. The nozzles to be actuated during top edgeprinting are selected appropriately so that each raster line is servicedat least once with each of the two drive signal waveforms. By so doing,it is possible to effect printing on the raster lines, which are printedby top edge printing, using a combination of the multi-shot dot seriesand the variable dot series, in the same manner as the raster lines thatare printed by medial printing alone.

[0120]FIGS. 21 and 22 show scanning parameters for bottom edge printingin the embodiment and the nozzles employed for printing each raster lineduring each pass. In the tables given in FIGS. 21 and 22, pass 0represents the final main scan. Thus, pass −11, for example, representsthe eleventh-to-last pass before final pass 0. Pass −5 through pass 0represent the six passes constituting bottom edge printing. In pass −5,the initial pass in bottom edge printing, the sub-scan feed L is 15dot-pitches, but sub-scan feed L assumes a constant value of 5dot-pitches from pass −4 through pass 0. In the initial pass in bottomedge printing (pass −5), the variable dot drive signal waveform is usedin the forward pass, and the variable dot drive signal waveform is alsoused in the forward pass during the medial printing pass just preceding(pass −6) as well. Accordingly, a reverse pass during which no dots areprinted is inserted between these two passes.

[0121] In FIG. 22, the raster line denoted by raster number 0 is theraster line situated at the bottom edge of the printable area. Thenegative raster numbers assigned to the other raster lines represent thenumber of the raster line counting from the bottom edge raster line. Inbottom edge printing as well, drive signal waveform and the nozzles tobe actuated are selected on a per-pass basis. As a result, each rasterline can be printed with a combination of the multi-shot dot series andthe variable dot series. By performing bottom edge printing in this wayit is possible to expand the effective printable range.

[0122] As will be apparent from the preceding description, according tothe present embodiment, drive signal waveform for each pass during topedge and bottom edge printing is selected in such a way that each rasterline is serviced at least once with each of the two drive signalwaveforms. In this way, it is possible to effect printing on the rasterlines, which are printed by top edge or bottom edge printing, using acombination of the multi-shot dot series and the variable dot series, inthe same manner as the raster lines that are printed by medial printingalone, thereby achieving high print quality.

[0123]FIGS. 23 and 24 depict a modification of bottom edge printing inthe present embodiment. The bottom edge printing depicted in FIG. 24differs from that depicted in FIGS. 21 and 22 simply in terms of theforward pass and reverse pass assignments. Specifically, during thepasses of the bottom edge printing (pass −5 through pass 0), thevariable dot series is used in the forward pass and the multi-shot dotseries is used in the reverse pass. Accordingly, main scan speed in theforward pass is 200 cps, which is suitable for the variable dot series,and in the reverse pass it is 250 cps, which is suitable for themulti-shot dot series.

[0124] In this modification, the final pass of medial printing takesplace in the reverse pass, and the initial pass of bottom edge printingtakes place in the forward pass. Accordingly, there is no need to insertan additional non-printing pass between these two passes. The speed ofbottom edge printing is thus improved somewhat relative to that depictedin FIGS. 21 and 22. Typically, by reversing the forward/reversedirections of the main scan passes for the respective drive signalwaveforms during medial printing relative to those during bottom edge(or top edge) printing, the speed of bottom edge (or top edge) printingmay be improved somewhat relative to the case where these are notreversed. In other words, performing each main scan at a main scanningspeed appropriate to the maskable drive signal selected for each of theforward and reverse passes gives a higher degree of freedom in terms ofusing the maskable drive signals, thereby effecting better printing.

F. Other Modification Examples F1. Modification Example 1

[0125] The preceding embodiment describes the use of both top edgeprinting and bottom edge printing; however, it would be possible to useone or other alone as needed.

F2. Modification Example 1

[0126] In the preceding embodiment, printing is performed in twodirections; however, the present invention is applicable tounidirectional printing as well. In this case, both the multi-shot dotseries and the variable dot series would be printed in the forward pass.

F3. Modification Example 3

[0127] In the preceding embodiment, two maskable drive signals, one formulti-shot dots and one for variable dots, were used concomitantly;however, it is generally possible to effect printing of a page using anarbitrary number n (where n is an integer equal to 2 or greater) ofmaskable drive signals. Main scanning speed may be set a levelappropriate for each maskable drive signal. Where main scanning speedmay assume any of a number of different values, various maskable drivesignal waveforms can be used to enable printing with various sets of dotseries.

[0128] In the preceding embodiment, printing of each raster line iscompleted in two scan passes, one using the multi-shot dot drive signalwaveform and one using the variable dot drive signal waveform. It would,however, be possible to complete printing of each raster line by meansof m (where m is an integer equal to 1 or greater) main scan passesusing each drive signal waveform m times. Generally, printing on eachraster line may be completed in n×m main scan passes using each of nmaskable drive signals m times (where n is an integer equal to 2 orgreater, and m is an integer equal to 1 or greater). For the rasterlines which are printed by bottom edge printing or top edge printing,printing on each raster line may be completed in at least n×m main scanpassess using each of n maskable drive signals at least m times.

[0129] If m is equal to 2 or greater, each of the m main scan passesperformed on a given raster line using a given maskable drive signalwill preferably target for printing one of every m intermediate pixellocations.

F4. Modification Example 4

[0130] The present invention may be implemented in a drum scan printeras well. In a drum scan printer, the direction of drum rotation is themain scanning direction and the direction of carriage travel is thesub-scanning direction. The present invention is applicable not only toink-jet printers specifically, but more generally to any types ofprinting devices wherein the surface of a printing medium is printedwith a print head having a plurality of nozzles. Such printing devicesinclude inter alia facsimile devices and copying devices.

F5. Modification Example 5

[0131] Some of the elements implemented through hardware in thepreceding embodiment may be alternatively implemented through software,and, conversely, some of the elements implemented through software maybe replaced by hardware. For example, some of the functions of thesystem controlled 54 (FIG. 2) could be executed by the host computer100.

[0132] A computer program for executing these functions may be providedin the form of a computer-readable storage medium such as a floppy diskor CD-ROM. The host computer 100 will then read the computer programfrom the storage medium and transfer it to an internal memory device orexternal memory device. Alternatively, the computer program may beprovided to the host computer 100 from a program provider device via acommunications link. The functions of the computer program will beexecuted by the microprocessor of the host computer 100 on the basis ofthe computer program stored in an internal memory device. Alternatively,the computer program may be executed directly from the storage medium bythe host computer 100.

[0133] As used herein, host computer 100 includes both hardware devicesand an operating system, and refers to hardware devices operated underthe control of the operating system. The functions of the variouscomponents of the host computer 100 are executed on the basis of thecomputer program. Some of the above-described functions may be providedvia the operating system, rather than applications.

[0134] As used herein, “computer-readable storage medium” is not limitedto portable storage media such as flexible disks and CD-ROMs, butincludes internal storage devices such as RAM or ROM installed withinthe computer, or external storage devices such as a hard disk fixed inthe computer.

[0135] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A printing apparatus for printing ink dots on aprint medium during main scan passes, comprising: a print head having aplurality of nozzles and a plurality of ink-expulsion drive elements forjetting ink droplets from each of the plurality of nozzles; a main scandriver for performing main scan by moving at least one of the printingmedium and the print head; a sub-scan driver for performing sub-scanfeed by moving one of the printing medium and the print head; a headdriver for presenting a drive signal to each of the ink-expulsion driveelements in response to a print signal; and a controller for controllingprinting operations, wherein the head driver includes: a maskable drivesignal generator capable of selectively generating for each main scanpass any of n types of maskable drive signals where n is an integerequal to 2 or greater; and a drive signal masking section that generatesthe drive signal to be supplied to the ink-expulsion drive elements, bymeans of masking the maskable drive signal according to the print signalon a per-pixel basis; and wherein the controller executes printing, onat least a part of the print medium, according to a specific printingscheme wherein printing ink dots on each raster line is completed in n×mmain scan passes while employing each of the n types of maskable drivesignals m times on each raster line where m is an integer equal to 1 orgreater.
 2. A printing apparatus according to claim 1, wherein at leastone of the n types of maskable drive signals can effect printing at aprint resolution different from that produced by other maskable drivesignals when used alone for printing; and when a printing operation isperformed using the n types of maskable drive signals, the printing ofink dots and the sub-scan feed are carried at in units of pixel pitchesconforming to a lowest print resolution of the print resolutionsachievable by the respective n types of maskable drive signals.
 3. Aprinting apparatus according to claim 2, wherein the print head iscapable of producing a plurality of dot types of different size for atleast one ink color on a print medium using the nozzles; the printsignal contains multiple bits per pixel so as to allow each pixel to beprinted in multi tones; each of the n types of maskable drive signalsincludes a plurality of pulses during each pixel interval; and the drivesignal masking section masks the maskable drive signals responsive tothe multiple-bit print signal.
 4. A printing apparatus according toclaim 2, wherein the printing apparatus can perform bidirectionalprinting where the printing of ink dots takes place in both forward andreverse passes; and different maskable drive signals are selected forthe forward pass and the reverse pass of main scan.
 5. A printingapparatus according to claim 2, wherein the printing apparatus canperform bidirectional printing where the printing of ink dots takesplace in both forward and reverse passes; one of the n types of maskabledrive signals is selected for each main scan pass; and the main scandriver performs each main scan at a speed appropriate to the selectedmaskable drive signal.
 6. A printing apparatus according to claim 1,wherein the main scan driver, when conducting main scan passes using atleast one specific maskable drive signal from among the n types ofmaskable drive signals, conducts the main scan at a speed different fromthat of main scan conducted using other maskable drive signals.
 7. Aprinting apparatus according to claim 6, wherein at least one of the ntypes of maskable drive signals can effect printing at a printresolution different from that produced by other maskable drive signalswhen used alone for printing; and when a printing operation is performedusing the n types of maskable drive signals, the printing of ink dotsand the sub-scan feed are carried at in units of pixel pitchesconforming to a lowest print resolution of the print resolutionsachievable by the respective n types of maskable drive signals.
 8. Aprinting apparatus according to claim 6, wherein the print head iscapable of producing a plurality of dot types of different size for atleast one ink color on a print medium using the nozzles; the printsignal contains multiple bits per pixel so as to allow each pixel to beprinted in multi tones; each of the n types of maskable drive signalsincludes a plurality of pulses during each pixel interval; and the drivesignal masking section masks the maskable drive signals responsive tothe multiple-bit print signal.
 9. A printing apparatus according toclaim 6, wherein the printing apparatus can perform bi-directionalprinting where the printing of ink dots takes place in both forward andreverse passes; and different maskable drive signals are selected forthe forward pass and the reverse pass of main scan.
 10. A printingapparatus according to claim 6, wherein the printing apparatus canperform bi-directional printing where the printing of ink dots takesplace in both forward and reverse passes; one of the n types of maskabledrive signals is selected for each main scan pass; and the main scandriver performs each main scan at a speed appropriate to the selectedmaskable drive signal.
 11. A printing apparatus according to claim 1,wherein the controller executes printing according to a first printingscheme in a medial section of a printable area of the print medium,while in at least one of a leading edge portion and trailing edgeportion of the printable area, executes printing according to a secondprinting scheme whose amount of the sub-scan feed is smaller than in thefirst printing scheme; and wherein the controller, with respect toraster lines printed according to the first printing scheme alone,completes printing in the n×m main scan passes employing each of the ntypes of maskable drive signals m times; and wherein the controller,with respect to raster lines printed according to both the firstprinting scheme and the second printing scheme, selects the maskabledrive signal for each main scan pass according to the second printingscheme such that at least n×m main scan passes are performed employingeach of the n types of maskable drive signals at least m times on eachof the raster lines.
 12. A printing apparatus according to claim 11,wherein at least one of the n types of maskable drive signals can effectprinting at a print resolution different from that produced by othermaskable drive signals when used alone for printing; and when a printingoperation is performed using the n types of maskable drive signals, theprinting of ink dots and the sub-scan feed are carried at in units ofpixel pitches conforming to a lowest print resolution of the printresolutions achievable by the respective n types of maskable drivesignals.
 13. A printing apparatus according to claim 11, wherein theprint head is capable of producing a plurality of dot types of differentsize for at least one ink color on a print medium using the nozzles; theprint signal contains multiple bits per pixel so as to allow each pixelto be printed in multi tones; each of the n types of maskable drivesignals includes a plurality of pulses during each pixel interval; andthe drive signal masking section masks the maskable drive signalsresponsive to the multiple-bit print signal.
 14. A printing apparatusaccording to claim 11, wherein the printing apparatus can performbidirectional printing where the printing of ink dots takes place inboth forward and reverse passes; and different maskable drive signalsare selected for the forward pass and the reverse pass of main scan. 15.A printing apparatus according to claim 11, wherein the printingapparatus can perform bi-directional printing where the printing of inkdots takes place in both forward and reverse passes; one of the n typesof maskable drive signals is selected for each main scan pass; and themain scan driver performs each main scan at a speed appropriate to theselected maskable drive signal.
 16. In a printing apparatus comprising aprint head having a plurality of nozzles and a plurality ofink-expulsion drive elements for expelling ink droplets from each of theplurality of nozzles, and a head driver for masking a maskable drivesignal to produce a drive signal to be supplied to each of theink-expulsion drive elements, a printing method comprising the steps of:executing printing, on at least a part of the print medium, according toa specific printing scheme wherein printing ink dots on each raster lineis completed in n×m main scan passes while employing each of n types ofmaskable drive signals m times on each raster line where n is an integerequal to 2 or greater and m is an integer equal to 1 or greater.
 17. Aprinting method according to claim 16, wherein at least one of the ntypes of maskable drive signals can effect printing at a printresolution different from that produced by other maskable drive signalswhen used alone for printing; and when a printing operation is performedusing the n types of maskable drive signals, the printing of ink dotsand the sub-scan feed are carried at in units of pixel pitchesconforming to a lowest print resolution of the print resolutionsachievable by the respective n types of maskable drive signals.
 18. Aprinting method according to claim 17, wherein the print head is capableof producing a plurality of dot types of different size for at least oneink color on a print medium using the nozzles; the print signal containsmultiple bits per pixel so as to allow each pixel to be printed in multitones; each of the n types of maskable drive signals includes aplurality of pulses during each pixel interval; and the maskable drivesignals is masked responsive to the multiple-bit print signal.
 19. Aprinting method according to claim 17, wherein the printing is performedbi-directionally where the printing of ink dots takes place in bothforward and reverse passes; and different maskable drive signals areselected for the forward pass and the reverse pass of main scan.
 20. Aprinting method according to claim 17, wherein the printing is performedbi-directionally where the printing of ink dots takes place in bothforward and reverse passes; one of the n types of maskable drive signalsis selected for each main scan pass; and each main scan pass isperformed at a speed appropriate to the selected maskable drive signal.21. A printing method according to claim 16, wherein when main scanpasses are conducted using at least one specific maskable drive signalselected from among the n types of maskable drive signals, the main scanis performed at a speed different from that of main scan conducted usingother maskable drive signals.
 22. A printing method according to claim21, wherein at least one of the n types of maskable drive signals caneffect printing at a print resolution different from that produced byother maskable drive signals when used alone for printing; and when aprinting operation is performed using the n types of maskable drivesignals, the printing of ink dots and the sub-scan feed are carried atin units of pixel pitches conforming to a lowest print resolution of theprint resolutions achievable by the respective n types of maskable drivesignals.
 23. A printing method according to claim 21, wherein the printhead is capable of producing a plurality of dot types of different sizefor at least one ink color on a print medium using the nozzles; theprint signal contains multiple bits per pixel so as to allow each pixelto be printed in multi tones; each of the n types of maskable drivesignals includes a plurality of pulses during each pixel interval; andthe maskable drive signals is masked responsive to the multiple-bitprint signal.
 24. A printing method according to claim 21, wherein theprinting is performed bi-directionally where the printing of ink dotstakes place in both forward and reverse passes; and different maskabledrive signals are selected for the forward pass and the reverse pass ofmain scan.
 25. A printing method according to claim 21, wherein theprinting is performed bi-directionally where the printing of ink dotstakes place in both forward and reverse passes; one of the n types ofmaskable drive signals is selected for each main scan pass; and eachmain scan pass is performed at a speed appropriate to the selectedmaskable drive signal.
 26. A printing method according to claim 16,wherein the printing is executed according to a first printing scheme ina medial section of a printable area of the print medium, while in atleast one of a leading edge portion and trailing edge portion of theprintable area, the printing is executed according to a second printingscheme whose amount of the sub-scan feed is smaller than in the firstprinting scheme; with respect to raster lines printed according to thefirst printing scheme alone, the printing of ink dots is completed inthe n×m main scan passes employing each of the n types of maskable drivesignals m times; and with respect to raster lines printed according toboth the first printing scheme and the second printing scheme, themaskable drive signal is selected for each main scan pass according tothe second printing scheme such that at least n×m main scan passes areperformed employing each of the n types of maskable drive signals atleast m times on each of the raster lines.
 27. A printing methodaccording to claim 26, wherein at least one of the n types of maskabledrive signals can effect printing at a print resolution different fromthat produced by other maskable drive signals when used alone forprinting; and when a printing operation is performed using the n typesof maskable drive signals, the printing of ink dots and the sub-scanfeed are carried at in units of pixel pitches conforming to a lowestprint resolution of the print resolutions achievable by the respective ntypes of maskable drive signals.
 28. A printing method according toclaim 26, wherein the print head is capable of producing a plurality ofdot types of different size for at least one ink color on a print mediumusing the nozzles; the print signal contains multiple bits per pixel soas to allow each pixel to be printed in multi tones; each of the n typesof maskable drive signals includes a plurality of pulses during eachpixel interval; and the maskable drive signals is masked responsive tothe multiple-bit print signal.
 29. A printing method according to claim26, wherein the printing is performed bi-directionally where theprinting of ink dots takes place in both forward and reverse passes; anddifferent maskable drive signals are selected for the forward pass andthe reverse pass of main scan.
 30. A printing method according to claim26, wherein the printing is performed bi-directionally where theprinting of ink dots takes place in both forward and reverse passes; oneof the n types of maskable drive signals is selected for each main scanpass; and each main scan pass is performed at a speed appropriate to theselected maskable drive signal.